Related Stories

Nobody predicts this surprising result and few guess what's actually going on but everybody wants to know. Try it with a room full of adults and watch the theories fly.

1. If you're doing this with toddlers, start at the beginning. Grab some kitchen ingredients and investigate things that dissolve in water and things that don't. Try sugar, salt, flour, popcorn and anything else you can find. (Note: adults might want to revisit this step once they've seen what the chocolate buttons do.)

2. Next, try a single coloured sugar-coated chocolate button. Fill a shallow dish with water (a white peanut butter jar lid is perfect). Let the water settle for at least 30 seconds, then pop the chocolate in the centre of the dish and watch what happens.

3. Wait long enough and you'll see this surprising result. The shell's colour migrates to the perimeter of the dish while the chocolate centre remains undissolved.

4. Start again with fresh water but this time, place four sugar-coated chocolates around the perimeter of the dish. Can you predict what will happen when the colours from each one meet?

5. Common sense leads most people to expect the colours to mix resulting in orange where red and yellow meets, and green where the blue and yellow mix. Few people guess that this is what happens instead…

6. Now that theories are flying around the room, you might like to see what happens to a drop of food colour in a dish of water.

7. If the water is very cold, it hardly moves at all. If the water is very warm, it spreads much faster. But it never migrates to the perimeter like the colour from a single sugar-coated chocolate button. How are those theories looking now?

What's going on?

Believe it or not, this little experiment can teach you a lot about a little-known ocean current and how dissolved gases manage to find their way to the bottom of the sea. That's a gigantic leap from a few sugar-coated choclates in a tiny dish of water so let's start from the beginning.

When sugar or salt or any soluble substance that sinks in water dissolves, the substance vanishes but it's still actually there. Apart from tasting sweet or salty, the solution has another characteristic we rarely consider. It is denser than the pure water you started with. Pour your solution into pure water and it will sink but because both are clear, you won't notice unless you're paying very close attention.

So what's going on with that single coloured sugar-coated chocolate button? It helps if you know a bit about the anatomy of those cheery chocolate treats that (supposedly) melt in your hand and not in your mouth.

The outer layer is almost entirely made of sugar. The majority of this layer is white and only the outermost part of it is coloured. Oh, and don't forget that sugar is actually a clear, crystalline substance, and not really 'white'. A bowl of sugar looks white because all the grains reflect white light in every direction, but pop a single grain under a microscope and it's clear, like glass.

It also helps if you try the experiment with a single drop of food colour in a dish of cold water. You'll notice it does very close to nothing, so we can safely say that the food colour in the chocolate's shell is not causing the surprising result. (You would see something quite different in warmer water, but we'll come back to that later.)

Pop a sugar-coated chocolate button into a shallow dish of water and what you're seeing is the outer layer of sugar dissolving. As you know, the sugar solution produced is denser than pure water, so it sinks and then spreads out. Scientifically speaking, what you see is a density gradient forming between the sugar solution in the centre of the dish and the pure water around the perimeter.

This density gradient causes a current — yes, like an ocean current — which flows away from the higher sugar concentration in the centre towards the area of low sugar concentration at the rim.

Once the outer layer of coloured sugar has dissolved, the density gradient continues to push sugary water towards the perimeter of the dish. Without food colour to make this stage of the proceedings visible, the colour looks like it's magically attracted to the rim but it's actually the remaining, uncoloured sugar dissolving into the water that's making the magic happen.

So far, so good, I hope. But what's happening with the four chocolates?

You'll notice the colour spreads out and away from each chocolate in a semi-circular looking fashion. Where they meet, the colourful currents appear to abruptly stop dead in their tracks.

The reason for the sudden pause in proceedings is, again, all to do with density gradients. Initially, the currents spread in every direction. But at the junction where any two colours meet, the concentration of sugar (that is, the density of the solution) is equal on both sides. With no difference in the density of the solution on either side, the density-driven current stops.

It doesn't really stop though. More sugar keeps dissolving away from each chocolate and piling up at these junctions but you don't notice because sugar dissolves clear. If you carefully measured the sugar concentration on either side of a junction, you would see it rising until all the sugar has dissolved.

Ocean currents

So what's this experiment got to do with ocean currents? Well, nobody is chucking continent-sized chocolate buttons into the sea, but there is a gigantic density-driven current most people have never even heard of.

It's called the thermohaline circulation and it is driven by differences in salinity and temperature, both of which affect the density of seawater.

Let's start at the South Pole. When Antarctic waters begin to freeze at the start of autumn, the seawater's salinity begins to change. To understand why, I need to take you on a quick conceptual detour away from dissolving chocolates to frozen cordial cups.

If you've ever made one of these summertime treats, you'll recall that the sweetest, yummiest bit is always at the bottom. Every kid soon learns to flip frozen cordial cups upside down because the most delicious bit is always at the bottom. Few people ever figure out why but here's what's going on.

Water molecules bond with each other at 0° Celsius but they won't bond with impurities until they reach lower temperatures. Your freezer compartment is 15–20° below zero, cold enough to freeze a cordial solution. But because the extremities cool down first, things aren't all that simple.

As the surface reaches the freezing point, those water molecules begin to bond with each other to form ice crystals but they reject the impurities, that is, sugar and food colour. The sugar concentration of the cordial just under the icy surface is slightly increased because of the rejection of impurities by those freshly frozen water molecules above.

It's also a bit colder than the cordial deeper within the cup. So this layer of cordial is slightly sweeter and colder which combine to make it more dense than the cordial just below.

As a result, this layer just below the icy surface begins to sink. As more freezing occurs, more slightly denser and slightly colder cordial is produced, which also sinks. So for nearly the whole time that your cordial cup is freezing, a density-driven current is slowly transporting more sugar to the bottom of the cup. Nifty!

Now, back to Antarctica. When the sea surface begins to freeze in autumn, those freezing water molecules reject dissolved impurities too but here, it's mainly salt (not sugar). A saltier and slightly colder layer of water forms just under the ice which sinks to the ocean floor, just like the sweeter water in your cordial cup.

Surface water flows in to replace the sinking water and as more and more of it freezes, more and more of this saltier, colder water descends into the ocean's depths. The volume of sinking water is truly colossal and it plays a crucial role in huge number of ocean processes.

This sinking water doesn't just produce currents. It also delivers vast quantities of dissolved oxygen to the ocean floor. Oxygen gets into water by surface mixing which is driven by waves whipped up by the wind. But it's the freezing of seawater that causes the sinking that delivers this vital oxygen to the ocean floor. Without that oxygen, there'd be no deep sea fish.

The freezing of Arctic and Antarctic waters is only part of the amazing story of thermohaline circulation. The shape of the ocean floor and positions of the Earth's continents, plus the daily rotation of the planet and the upwelling caused by surface winds all combine to keep a gigantic conveyor belt of water moving right around the planet.

Problem is, I'm fast running out of space to keep writing but, rest assured, I'm now busily looking for another kitchen science experiment to help tell the rest of this amazing story. Till then, have fun with your chocolates and don't, whatever you do, eat too many!